In recent years, reduction in fuel consumption of cars is increasingly required, and tires having small rolling resistance are required. Accordingly, a rubber composition which is excellent in a low hysteresis loss property and a low heat build-up is required as a rubber composition used for a tread and the like in a tire. Further, excellent abrasion resistance and excellent tensile strength in addition to a low hysteresis loss property are required in a rubber composition for a tread. In contrast with this, it is effective for improving a low hysteresis loss property, abrasion resistance and tensile strength of a rubber composition to enhance affinity of fillers such as carbon black and silica with a rubber component in the rubber composition.
In order to enhance affinity of fillers with a rubber component in a rubber composition to improve a reinforcing effect exerted by the filler, developed are, for example, synthetic rubbers which are improved in affinity with fillers by modifying an end of molecular chain and synthetic rubbers which are improved in affinity with fillers by copolymerizing with functional group-containing monomers.
On the other hand, in respect to natural rubber, techniques in which, for example, a vinyl base monomer is added to natural rubber latex and subjected to graft polymerization therewith are known (refer to patent documents 1 to 6), and grafted natural rubbers obtained by the techniques are put into practical use in applications of adhesives and the like. However, since the such grafted natural rubbers are grafted with a large amount (20 to 50% by mass) of a vinyl compound as a monomer, they increased in a viscosity to a large extent when blended with fillers such as carbon black and silica, and processability becomes worse. Further, since a large amount of the vinyl compound is introduced into a molecular chain of the natural rubber, the characteristics of the natural rubber itself are changed, and the excellent physical characteristics (viscoelasticity, a strain-stress curve in a tensile test and the like) which are intrinsic to the natural rubber become worse. Accordingly, even use of the grafted natural rubbers obtained by the techniques does not make it possible to improve affinity thereof with fillers to enhance a reinforcing effect thereof. Further, it is proposed to use an epoxidized natural rubber in order to improve flex cracking resistance and strength of a tire (refer to patent document 7).
In contrast with this, disclosed is a technique in which a polar group-containing monomer is added to natural rubber latex to subject the polar group-containing monomer to graft polymerization with a natural rubber molecule in the natural rubber latex and in which the modified natural rubber coagulated and dried is used as a rubber component to thereby enhance affinity of the rubber component with fillers to improve a reinforcing property of the rubber composition and enhance a low hysteresis loss property, an abrasion property and tensile strength of the rubber composition (refer to patent document 8). In recent years, however, it is required to further enhance a low hysteresis loss property and an abrasion property of a rubber composition.    Patent document 1: Japanese Patent Application Laid-Open Hei 5 No. 287121    Patent document 2: Japanese Patent Application Laid-Open Hei 6 No. 329702    Patent document 3: Japanese Patent Application Laid-Open Hei 9 No. 25468    Patent document 4: Japanese Patent Application Laid-Open No. 2000-319339    Patent document 5: Japanese Patent Application Laid-Open No. 2002-138266    Patent document 6: Japanese Patent Application Laid-Open No. 2002-348559    Patent document 7: Japanese Patent Application Laid-Open No. 2007-56205    Patent document 8: International Publication No. 2004-106397